Omgnb - Should i ask for the compensation because of the shop out of stock of my orders?

Few days ago, i was placing an order on the website named omgnb, and i bought school backpack and one canvas backpack from two of their brands, "MXM" and the "SCCOTTON", and there is nothing unusual while i was ordering or paying.

However, on the next day morning, i receive an email from the seller, and it said that, one of my order lists backpack was out of stock!!! And they told me that whether agree with their refund to me or choose another backpack instead.

So that is the case, it is obvious that is their mistake lead to the case. And what i found is that school backpack there still show in stock!!!

Should i ask for the compensation? Or just take the refund of that bag? Actually, i'm really not happy with the state of that backpack is still in stock, although they respond quickly and agree to refund to me.